Known photoconductive materials for electro-photographic light-sensitive compositions include various organic compounds some of which are known to compounds have considerably high sensitivity. However, it is very rare at persent to practically use organic photoconductive materials for electrophotographic materials.
Organic photoconductive materials for photoconductors have various excellent properties as compared with inorganic photoconductive materials or photoconductors and hence provide a wide range of technical application in the technical field of electrophotography. For example, the production of transparent electrophotographic light-sensitive films, flexible electrophotographic light-sensitive films, light-weight and easily handling electro-photographic light-sensitive films, etc., have become possible for the first time using organic photoconductors. Also, organic photoconductors have such properties as a film-forming property during the production of electro-photographic light-sensitive materials, a surface smoothness, and a selectivity of a charging polarity for electro-photographic processes. Inorganic photoconductors do not have such properties.
In spite of having various excellent properties, the organic photoconductors have not sufficiently contributed to the technical field of electrophotography up to now mainly because of their low light-sensitivity and the brittleness of the films or layers of the photoconductors.
The study of organic photoconductors was first made on compounds such as low molecular heterocyclic compounds, nitrogen-containing aromatic compounds, and various high molecular aromatic compounds. As the results of such studies, some compounds having a considerably high sensitivity were found. However, recently a sensitization for obtaining higher sensitivity is likely to become the center of study. This is because even organic photo-semiconductive compounds do not have sufficient sensitivity such that they can be used without the need of the application of a sensitizing treatment. Therefore, when practically using an organic photoconductor, it is necessary to select the most effective sensitizing method and apply the sensitization to organic photoconductors. Therefore, it is clear that the industrial value of an organic photoconductor depends upon the extent of the sensitivity of the electrophotographic light-sensitive material finally obtained by a sensitizing means applied.
The most generally known method such as a sensitizing method is the addition of a sensitizing dye and the addition of a Lewis acid, which can be applied to almost all organic photoconductors. The former method sensitizes the organic photoconductor by imparting the spectral absorption characteristics of a dye to the organic photoconductor and the latter method sensitizes the organic photoconductor by forming a complex of donor and acceptor in the organic photoconductor to provide a new spectral sensitivity.